Synergistic effects of hygrothermal conditions and solar ultraviolet radiation on the properties of structural particulate-filled epoxy polymer coatings

https://doi.org/10.1016/j.conbuildmat.2021.122336Get rights and content

Highlights

  • Properties of filled epoxy coating exposed to moisture, elevated temperature and UV.

  • Durability of particulate-filled epoxy polymer coatings.

  • Surface microcracks after UV exposure that were wider with hygrothermal conditioning.

  • Environmental conditioning enhanced the properties of coatings with at least 40% filler content.

Abstract

The synergistic effects of solar ultraviolet (UV) radiation, moisture, and in-service temperature on the properties of structural particulate-filled epoxy-based polymer coatings were investigated. The coatings contained up to 60% by volume of hydrated alumina powder and fly ash as fillers. Four sets of coating specimens (20 samples per set) with dimensions of 60 mm × 10 mm × 5 mm were prepared. Two sets were conditioned at a relative humidity of 98% and temperature of 60 °C for 2000 hrs (HG). One of these sets was then exposed to simulated UV conditions for 2000 hrs with the other set evaluated for the effect of HG conditioning. One set was unconditioned and served as control specimens with another set exposed to UV. Physical observations showed yellowing on the surface of neat epoxy coating after HG or UV exposure, but the presence of fillers minimized fading and weight loss. Regardless of conditioning environment, there was no reduction in the flexural strength for the polymer coatings containing at least 40% fillers. HG or UV exposure promoted post-curing, increased the glass transition temperature, and enhanced cross-linking density. Microscopic observation revealed the formation of surface microcracks after UV exposure that were wider with HG conditioning. ANOVA showed that the combination of HG and solar UV radiation negatively impacted the flexural properties of the coatings with up to 20% filler content but enhanced the coating properties with filler contents above 40%.

Introduction

Epoxy-based polymers are currently being applied as protective coatings to minimise the environmental effects on the properties of composite structures [1], [2]. In this application, the structural polymer coatings are exposed to aggressive environmental conditions including solar ultraviolet (UV) radiation, moisture, and in-service elevated temperature. A number of studies have evaluated the sensitivity of polymers to hygrothermal conditioning [3], [4], [5] and found that the ingress of moisture can react with the epoxy polymer and impose internal stresses, causing a reduction in mechanical properties [6]. Elarbi and Wu [7] found that moderate to high heat could significantly degrade the mechanical properties of epoxy resins, but that low to moderate heat could promote post-curing resulting in enhanced flexural strength. Douglas et al. [8] suggested that fiber-reinforced polymer (FRP) bond is vulnerable to combined effects of high temperature and humidity/water. Ghasemi-Khahrizangi et al. [9] and Yusif and Haddad [10] highlighted the harmful effects of UV exposure on the physical, mechanical and chemical properties of epoxy polymers. The photo-oxidative reaction caused by solar UV radiation reduces molecular weight and makes the polymer more brittle [11] and can break the polymeric chains [12]. The degradation is due to the energy of solar UV light being higher than the chemical bond strength, e.g., C–C, O–O, H–O, and C–N in polymers [13]. Rosu et al. [14] also found that UV affects the polymer networks of epoxy resin and results in colour changes and mass loses in the exposed samples. Rezig et al. [15] investigated the relationship between chemical degradation and thickness loss of an unpigmented, non UV-stabilized, crosslinked amine-cured epoxy coating exposed to UV conditions and found that the rate of chemical degradation for an amine cured epoxy coating is always greater than that of the thickness loss. In another study, Lu et al.[16] concluded that UV degradation of polymeric surfaces was strongly dependent on UV wavelength, intensity and exposure time. Sung et al. [17] showed that intensity of Fourier-transform infrared spectroscopy (FTIR) peaks related to the CH stretching band at 2960 cm−1 (mass loss), NH bending and CN stretching at 1520 cm−1 (chain scission), and the C = O band at 1726 cm−1 in the epoxy coatings decreased as exposure time increased, indicating that degradation is an ablation process taking place in the outer surface. Chin et al. [18] observed surface erosion and cracking on thin polymer films made of vinyl ester and iso-polyester after exposure to a 1000-watt xenon arc source for 1200 h at 30 °C in an Oriel solar simulator. They found that deterioration of the mechanical properties of polymeric composites due to UV radiation is caused by the surface oxidation resulting in surface erosion and cracking of the polymer matrix. These studies showed the significant effect of UV on the durability properties of epoxy polymers. Attempts to reduce the transparency of polymer coating are therefore necessary to minimise their degradation under solar UV radiation.

A number of studies have added particulate fillers to epoxy resin system and investigated their short-term properties [19], [20], [21]. Ferdous et al. [20], [22] showed that the addition of light-weight filler materials including fly ash (FA), fire retardant (FR) and hollow microsphere (HM) to the resin minimised the cost of an epoxy polymer matrix. Similarly, Lokuge and Aravinthan [21] added fly ash to increase the compressive strength of epoxy-based polymer concrete. Research on the effects of different filler materials such as fly ash and silica fume on epoxy-based polymer concrete [21], [22], [23] have shown to improve their mechanical, chemical and durability properties. Golestaneh et al. [23] showed that a higher compressive strength can be achieved by adding fine or coarse filler to epoxy-based polymer concrete. The improvement in strength using a blend of fine and coarse filler was also reported by Lim et al. [24] where brittleness was reduced by optimising the filler contents. These studies have shown that the introduction of fillers enhanced the physical and mechanical properties of epoxy-based polymer matrix. However, the behaviour and long-term performance of structural epoxy coatings under different exposure conditions warrant detailed investigation.

Few studies have evaluated the long-term properties of epoxy matrices with fillers. Among these studies, Khotbehsara et al. [25] investigate the effect of in service elevated temperature on the mechanical properties and microstructure of particulate filled epoxy polymer including fly ash (FA) and hydrated alumina powder (FR). In another study, Khotbehsara et al. [26] investigated the effect of HG conditioning (1000, 2000 and 3000 h (hrs) at temperature up to 60 °C and a relative humidity of 98%) on the durability and service life of particulate-filled epoxy resin. The results showed that the inclusion of FA and FR fillers decreased the moisture absorption, increased the glass transition temperature and slightly reduced the mechanical properties of polymer coating. He et al. [27] found that the moisture absorption of DGEBA epoxy-based nanocomposites containing nano-calcium carbonate (nano-CaCO3) decreased with increasing nano-CaCO3 content. Shamsuddoha et al. [28] studied the long-term properties of epoxy polymers with different percentages (27%, 15% and 13%) and types of fillers (fine and coarse aggregates) under hot-wet conditioning for 1000 h at 70 °C. Their results showed a reduction of up to 74% and 83% in the compressive and flexural strengths, respectively. Tcherbi-Narteh et al. [29] incorporated nanoparticles into DGEBA epoxy resin to minimize the damaging effects of solar UV radiation on polymers. Their results showed that the addition of 1% by weight to the polymer delayed the onset of deleterious effects of UV due to partial curing of the epoxy and better dispersion of the nanoparticles. Cao et al. [30] detected some enhancement in the properties of polymer coating systems after 2000 hrs of UV exposure with the addition of titanium dioxide (TiO2) particles. In a recent study, Ferdous et al. [20] evaluated the role of particulate fillers including FA, HM and FR in the physical and flexural strength of epoxy polymers when exposed to simulated solar UV radiation. After 2000 hrs of exposure, they found embrittlement, discoloration, and weight loss in all the specimens. The weight loss (0.15%) was, however, lower in the specimens with the highest amount of fillers (60%) compared to neat epoxy resin (0.37%). These studies show that incorporating fillers in polymer coatings enhances UV resistance. Similar findings was observed by Khotbehsara et al. [31] wherein they found no reduction in the flexural strength for polymer coating containing at least 40% FA and FR fillers after 2000 hrs UV exposure. They also indicated that a thickness of at least 5 mm of epoxy-based structural coating with 60% by volume of particulate fillers could provide UV protection for 50 years. These studies show that incorporating fillers in polymer coatings enhances the durability of epoxy-based coating. However, these studies have focused on evaluating the long-term properties of polymeric materials exposed only to one weathering condition. In service conditions, materials are generally exposed to a combination of environmental conditions. It is necessary therefore to have a deep understanding on the synergistic effects of harsh environmental conditions on the long-term durability properties of particulate-filled epoxy polymer (PFR) coating.

This study presents an experimental investigation to determine the effect of solar UV radiation and hygrothermal conditioning on the properties of an epoxy-based coating system containing fire-retardant (FR) and fly-ash (FA) fillers. The significance of this study is to understand the degradation mechanisms related to the synergistic effects of solar UV radiation, elevated in-service temperatures, and high moisture levels on the physical, mechanical, and physicochemical properties and the microstructure of epoxy-based polymer coatings. Analysis of variance (ANOVA) was also conducted to statistically determine the synergistic effects of exposure to UV and HG environments as well as to evaluate which of the environmental conditions had the greatest effect on the properties of the epoxy-based polymer coatings.

Section snippets

Materials

Particulate-filled epoxy polymers were prepared by mixing resin and fillers. Part A of the resin system was DGEBA epoxy (bisphenol A diglycidyl ether) and Part B was an amine-based curing agent. Part A and part B were mixed together based on the Epoxy Equivalent Weight (EEW) of 190 g for Part-A and Amine Hydrogen Equivalent Weight (AHEW) of 60 g for Part-B as furnished by the supplier. The particulate fillers were fly ash (FA) and hydrated alumina powder (FR). The FA was suppled by Si Powders

Physical properties of the Epoxy-Based coatings

Fig. 3 shows the surface discoloration after exposure to 2000 hrs of HG conditioning and exposure to solar UV radiation. The first column (control samples) shows that the polymer coatings turned darker as the filler content increased, which would be expected due to the dark grey color of the fly ash. The second column shows that, after 2000 hrs of HG exposure, the F0 samples tended to be yellowish, but there were no significant changes in the samples with higher filler content. The

Microstructure

The microstructure of surface of F0, F20, F40 and F60 samples was observed under SEM (see Fig. 7). As show in Fig. 7, control samples F0 and F20 exhibited a denser microstructure with smaller pore sizes than its F40 and F60 counterpart, which had larger pores and a weak interfacial bond between the resin and fillers due to voids and pores in the matrix. Khotbehsara et al. [25] suggested that such voids and pores could reduce the flexural properties of highly filled polymer coatings compared to

Quantifying the influence of exposure condition and filler content with ANOVA

Analysis of variance (ANOVA) was used to determine the influence of the exposure conditions and filler content on the flexural strength of the particulate-filled epoxy polymers with SPSS statistical analysis software [45], [46], [47], [48]. Separate analyses were run for each filler content (0%, 20%, 40%, and 60%) and exposure condition (unconditioned control and HG, UV, or HG + UV conditioning). Table 3 provides the results of the one-way ANOVA (multiple comparisons in accordance with Tukey’s

Conclusion

This research assessed the physical, mechanical, thermomechanical, and microstructural characteristics of epoxy-based polymer coatings with different percentages of fly-ash and fire-retardant fillers. The coatings were exposed to hygrothermal (HG), solar UV radiation (UV), and combined hygrothermal and UV (HG + UV) environments. The following conclusions can be drawn from this study.

  • The resin-rich polymer coatings (F0 and F20) evidenced noticeable yellowing when exposed to UV or UV + HG

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The first author gratefully acknowledges the financial support—including fees, research scholarship, and stipend scholarship from the University of Southern Queensland (USQ) for conducting her doctorate. The authors are grateful to Dr. Barbara Harmes for her contribution.

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